Hopefully there are some Clevor fans out there!

Say I build my 408 Clevor with 2v heads, but want to switch to 4v heads later on. Will the holes I drill in the block for water passages match up with 4v heads as well as 2v heads?

 

I think the answer is yes.
http://www.themustangshop.com/clevor.cfm

But depending on your cam and heads, 2v may not flow enough for a 408.
4v with port plates may be better for you.